Exemplary embodiments of the present invention relate to processing of audio signals that are output as sound by audio listening devices. More specifically, exemplary embodiments relate to audio signal processing that is performed to reduce or mitigate audio fatigue resulting from use of audio listening devices.
With the invention of the professional in-ear, stage and studio, monitoring systems in the 1970's, large numbers of people began to experience the sealing of high fidelity speakers to the ear canal for the first time. These devices have protected hundreds of thousands of amateur and professional musicians and sound engineers from hearing loss due to excessive performance volumes. Nevertheless, in-ear monitors have been, and remain, a persistent source of audio fatigue and potential short term or long term hearing loss. They share this problem with other in-ear listening devices such as hearing aids, insert headphones, ear buds, and the like, as well as headphones and other over-ear devices that are designed to hold small loudspeakers in place close to a user's ears. Professional applications of in-ear monitors have called for some musicians and sound engineers to tolerate conditions of persistent audio fatigue, in which peoples' ears begin to physically ache or hurt after prolonged use of in-ear devices, which can become nearly intolerable by the end of a performance or recording session. These users often refer to a sensation of percussiveness beating their ears that cannot be eliminated by simply turning down the volume.
Almost every person has experienced a situation where the volume from another person's headphones could be heard even from across a room, on a bus, in a store, or any number of public venues. Given the sound volume necessary to be heard at a distance, the volume blasting directly into a listener's ear in such a situation can be expected to be excessive. The listener might be asked (or admonished) to turn down their headphones for the sake of their own health and for the courtesy to others, but what has not been hitherto widely realized is that the person listening to the headphones has already, unknowingly turned down their own personal perception of the volume through a natural hearing protection mechanism known as the acoustic reflex. The acoustic reflex is a natural mechanism involving an involuntary muscle contraction that occurs in the middle ear in response to high-intensity sound stimuli. By way of the acoustic reflex, contraction of the stapedius muscle in the ear reduces the ear's sensitivity in order to protect itself from being damaged by loud noises and to widen its dynamic range to higher sound pressure levels.
The persistent triggering of this reflex by insert headphones, hearing aids, and the like, may perpetuate a cycle in which the user continually increases the volume to counteract the effects of the acoustic reflex. This can also set up an additional dangerous situation for a user who is already tolerating very loud volumes due to the acoustic reflex and accidentally or intentionally turns the volume up even further at a point at which the stapedius muscle has already become exhausted or reached a limit of its inherent ability to protect the ear from loud sounds and temporary or even permanent hearing loss.